This invention is particularly concerned with fluorescent lamps having more than one phosphor layer, for example, two phosphor layers, one superposed on the other. Such superposed double layers have been used to improve maintenance or to reduce phosphor cost and are shown in U.S. Pat. Nos. 3,602,757, 3,602,758 4,070,598 and 4,088,923.
In U.S. Pat. Nos. 3,602,757 and 4,070,598 the first or outer phosphor layer provides a desired color of light but has low lamp maintenance. The second or inner layer has a high maintenance and serves to screen the outer layer from damaging short wavelength radiation from the arc discharge, thereby improving lumen maintenance of the phosphor in the outer layer.
In U.S. Pat. No. 3,602,758, the outer phosphor layer comprises relatively inexpensive material while the inner phosphor layer comprises more expensive material. This arrangement permits use of less of the expensive material necessary to provide a desired spectral energy distribution than if both materials had been blended and applied as a single layer.
U.S. Pat. No. 4,088,923 also involves use of an outer layer of inexpensive phosphor and an inner layer of more expensive phosphor. The patent is concerned with the color of the light emitted by the lamp and discloses that the inner layer should be sufficiently thick to convert at least 80% of the ultraviolet radiation from the arc discharge into light.
In my invention the primary consideration is the improvement of maintenance of a fluorescent lamp by providing an underlying additional phosphor layer beneath that of the overlying desired phosphor.